(1) Field of the Invention
The present invention relates to a radar system, and especially relates to a construction of a receiving system of a transmitting-receiving module used for radar system.
(2) Description of the Related Art
In radar system, objects in the transmission direction of radio wave are detected by transmitting radio wave and receiving the reflected wave. In one type of radar systems, a phased array radar system, which detects objects in a predetermined range by changing the transmitting and receiving direction by changing phases of transmitting and receiving radio wave, is known.
In the radar system of this sort, it is necessary to provide a number of transmitting-receiving modules for transmitting and receiving radio wave.
The transmitting-receiving module used in the phased array radar system had a circuit configuration, for example, shown in FIG. 1. Namely, an antenna 1 for transmitting and receiving is connected with an A terminal of a three terminal circulator 2, an output of transmitting system power amplifier 3 is connected with a C terminal of the circulator 2, an A terminal of an RF switch 4 is connected with a B terminal of the circulator 2. And, a B terminal of the RF switch 4 is connected with an A terminal of an RF switch 5, a C terminal is connected with a terminating resistance 6 respectively.
And, a B terminal of the RF switch 5 is connected with a B terminal of an RF switch 7, a C terminal with a fixed attenuator 8 respectively, another terminal of the fixed attenuator 8 is connected with a C terminal of the RF switch 7. And, an A terminal of the RF switch 7 is connected with an input of a low noise amplifier 9, signal amplified by the low noise amplifier 9 is signal processed for example so as to be visual displayed in normal receiving circuit 10.
Operation of a conventional transmitting-receiving module having a circuit configuration mentioned above, is as follows. First, when in transmission, by connecting all of the RF switches 4, 5, 7 with C terminal side, reflected wave from antenna to the low noise amplifier 9 is minimized to prevent the low noise amplifier 9 from destruction.
Next, when receiving radar signals, by connecting all of the RF switches 4, 5, 7 with B terminal side, a maximum gain mode is obtained. Further, if an excessive receiving signal is supplied when receiving radar signal, the receiving circuit 10 detects it, and prevents saturation of the low noise amplifier 9 by connecting the RF switch 4 with B terminal side, the RF switch 5, 7 with C terminal side.
However, there was a problem that because in the above-mentioned transmitting-receiving module, three RF switches are connected in series from circulator 2 to low noise amplifier 9, so, noise figure of receiving system becomes high by the insertion loss of three RF switches.